a. Field of the Invention
The present invention relates to an apparatus for spectroscopy, specifically for high resolution particle energy loss spectroscopy of samples.
b. Description of the Prior Art
Energy analyzers and spectrometers based on electrostatic energy dispersion of electrons and other charged particles are widely used in basic and applied science. High-resolution electron energy loss spectroscopy (EELS) methods employ a low-energy (1-20 eV) electron beam to detect quantum energy losses due to intrinsic surface vibrations (phonons), vibrations of adsorbed atomic or molecular species of thin film samples, or molecular vibrations in gaseous samples.
Spectrometers employed for high-resolution EELS known in the art have commonly been based on cylindrical (127.degree.), spherical (180.degree.) or cylindrical mirror (42.degree.) analyzer designs. These instruments have been reviewed and their performance characteristics, including resolution and monochromatic current characteristics, have been discussed in Ibach and Mills, Electron Energy Loss Spectroscopy and Surface Vibrations (Academic, N.Y. 1982). Some prior art spectometers and methods purportedly have attained consistent system resolution of about 3.5 meV (measured in terms of energy width at half signal current, or abbreviated FWHM) in surface studies as described by Andersson and Persson in Phys. Rev. B 24, 3659 (1981) and by Lehwald and Ibach in Vibration at Surfaces, R. Caudano, J. M. Gilles and A. A. Lucas, eds. (Plenum, N.Y. 1982). Available data indicate that relatively high output currents at system resolutions of about 7-8 meV FWHM have been achieved by some spectrometers utilizing spherical designs, as described in J. E. Demuth, K. Christman and P. N. Sanda, Chem. Phys. Lett. 76, 201 (1980) and N. R. Avery, Appl. Surf. Sci. 13, 171 (1982). For the studies of thin films and of surface vibrations, it is desirable to maintain as high a monochromatic output current and pass energy at a given system resolution as possible. Heretofore, however, system resolution of about 2.5 meV at spectrometer pass energies of about 1.0 eV have not been obtainable by present spectrometers and known methods.
It is thus an object of the present invention to provide a spectrometer and methods for high resolution particle energy loss spectroscopy which obtain high monochromatic output currents from about 1.0 to 10.0.times.10.sup.-10 A while maintaining high energy spectrometer resolution between about 2.0 and 10 meV.
It is yet a further object of the present invention to provide an overall system resolution of about 3.0 meV while maintaining high signal levels and a spectrometer pass energy of about 1.0 eV. In addition, when lower pass energies are employed from about O.5 to about 1.0 eV, it is an object to maintain system resolution as high as about 2.5 meV.
It is yet another object of the present invention to provide a spectrometer utilizing a fixed geometry that affords compatibility with commercial surface analysis systems employing Auger electron spectroscopy (AES), photoelectron spectroscopy (ESCA), low-energy electron diffraction (LEED) and the like.